The present invention relates to a process for producing 2,3,5,6-tetrachloro-1,4-benzenedicarboxylic acid. More particularly, the present invention relates to a process for producing 2,3,5,6-tetrachloro-1,4-benzenedicarboxylic acid, wherein the formation of reaction by-products is less than in the conventional processes to thereby enable efficiently to produce 2,3,5,6-tetrachloro-1,4-benzenedicarboxylic acid.
Dimethyl 2,3,5,6-tetrachloro-1, 4-benzenedicarboxylate is widely and in a large amount used as an agricultural or horticultural herbicide.
A common process for producing dimethyl 2,3,5, 6-tetrachloro-1,4-benzenedicarboxylate comprises chlorinating benzene-1,4-dicarbonyl chloride to thereby obtain 2,3,5,6-tetrachloro-1,4-benzenedicarbonyl chloride and converting the same to a dimethyl ester with the use of methanol. However, this process has a drawback in that by-products such as hexachlorobenzene are formed in a large amount.
Therefore, there is a demand for the development of a process for producing dimethyl 2,3,5,6-tetrachloro-1,4-benzenedicarboxylate in which the content of impurities such as the above hexachlorobenzene is greatly reduced.
As the process for producing dimethyl 2,3,5, 6-tetrachloro-1,4-benzenedicarboxylate without the formation of hexachlorobenzene, there can be thought out a process comprising first performing an addition of water to 2,3,5,6-tetrachloro-1,4-benzenedicarbonitrile industrially readily procurable to thereby effect synthesis of 2,3,5,6-tetrachloro-1,4-benzenedicarboxamide, subsequently preparing 2,3,5,6-tetrachloro-1,4-benzenedicarboxylic acid from the 2,3,5,6-tetrachloro-1,4-benzenedicarboxamide and thereafter esterifying the 2,3,5,6-tetrachloro-1, 4-benzenedicarboxylic acid.
Apart from the above, generally, the method of performing an addition of water to a nitrile in sulfuric acid to thereby obtain a carboxamide and the method of hydrolyzing the carboxamide in sulfuric acid to thereby obtain a carboxylic acid are well known.
However, the hydrolysis of 2,3,5,6-tetrachloro-1,4-benzenedicarboxamide was difficult. Nevertheless, the inventors found that 2,3,5,6-tetrachloro-1, 4-benzenedicarboxamide can be hydrolyzed by heating the same in concentrated sulfuric acid at 220xc2x0 C. or higher temperatures. The hydrolysis of 2,3,5,6-tetrachloro-1,4-benzenedicarboxamide under such severe conditions was unfavorably accompanied by side reactions such as decarboxylation and sulfonation, thereby disenabling to obtain the desired compound, 2,3,5,6-tetrachloro-1, 4-benzenedicarboxylic acid, with a high yield.
Therefore, if a method of efficiently producing 2,3,5,6-tetrachloro-1,4-benzenedicarboxylic acid from 2,3,5,6-tetrachloro-1,4-benzenedicarboxamide is developed, its industrial value is extremely great.
An object of the present invention is to provide a process for producing 2,3,5,6-tetrachloro-1, 4-benzenedicarboxylic acid which is a precursor of dimethyl 2,3,5,6-tetrachloro-1,4-benzenedicarboxylate useful as an agricultural or horticultural herbicide. More particularly, an object of the present invention is to provide a process for producing 2,3,5, 6-tetrachloro-1,4-benzenedicarboxylic acid from 2,3,5, 6-tetrachloro-1,4-benzenedicarbonitrile or 2,3,5, 6-tetrachloro-1,4-benzenedicarboxamide as a feedstock, wherein the formation of reaction by-products is less than in the conventional processes to thereby enable the efficient production of 2,3,5,6-tetrachloro-l,4-benzenedicarboxylic acid.
In one aspect of the present invention, there is provided a process for producing 2,3,5,6-tetrachloro-1,4-benzenedicarboxylic acid, comprising heating 2,3,5,16-tetrachloro-1,4-benzenedicarboxamide at 110 to 190xc2x0 C. in the presence of sulfuric acid or fuming sulfuric acid containing water in an amount smaller than a stoichiometric value of hydrolysis of 2,3,5,6-tetrachloro-1,4-benzenedicarboxamide.
In another aspect of the present invention, there is provided a process for producing 2,3,5, 6-tetrachloro-1,4-benzenedicarboxylic acid, comprising heating 2,3,5,6-tetrachloro-1,4-benzenedicarbonitrile at 110 to 190xc2x0 C. in the presence of sulfuric acid or fuming sulfuric acid containing water in an amount smaller than a stoichiometric value required for forming 2,3,5,6-tetrachloro-1,4-benzenedicarboxylic acid from 2,3,5,6-tetrachloro-1,4-benzenedicarbonitrile.
The acidity function (xe2x80x94Ho) of the sulfuric acid or the sulfuric acid containing fuming sulfuric acid is preferably in the range of 10.27 to 14.44.
In the present invention, 2,3,5,6-tetrachloro-1, 4-benzenedicarboxylic acid can be obtained directly from industrially readily procurable 2,3,5,6-tetrachloro-1,4-benzenedicarbonitrile or from 2,3,5,6-tetrachloro-1,4-benzenedicarboxamide which can be easily produced therefrom, as a precursor, by heating the same at 110 to 190xc2x0 C. in the presence of sulfuric acid or fuming sulfuric acid containing water in an amount smaller than a stoichiometric value of hydrolysis. In the present invention, the hydrolysis of 2,3,5,6-tetrachloro-1,4-benzenedicarboxamide is advanced under mild conditions, so that the probability of side reactions is low. Therefore, the present invention enables to produce 2,3,5,6-tetrachloro-1, 4-benzenedicarboxylic acid more easily with a higher yield than in the prior art.
The process for producing 2,3,5,6-tetrachloro-1, 4-benzenedicarboxylic acid from 2,3,5,6-tetrachloro-1, 4-benzenedicarboxamide and the process for producing 2,3,5,6-tetrachloro-1,4-benzenedicarboxylic acid from 2,3,5,6-tetrachloro-1,4-benzenedicarbonitrile according to the present invention will be described separately from each other in detail below.
 less than 2,3,5,6-Tetrachloro-1,4-benzenedicarboxamide as a Feedstock greater than 
2,3,5,6-Tetrachloro-1,4-benzenedicarboxamide may be produced from 2,3,5,6-tetrachloro-1, 4-benzenedicarbonitrile. 2,3,5,6-Tetrachloro-1, 4-benzenedicarbonitrile is being produced on an industrial scale as a feedstock for drugs, agricultural or horticultural herbicides (common name: Tefluthrin, trade name: Force) and industrial chemicals, so that it is available at a low price. The reaction from 2,3,5,6-tetrachloro-1,4-benzenedicarbonitrile to 2,3,5,6-tetrachloro-1,4-benzenedicarboxamide proceeds in acid conditions readily and in a quantitative manner.
 less than Hydrolysis of 2,3,5,6-Tetrachloro-1, 4-benzenedicarboxamide greater than 
The hydrolysis of 2,3,5,6-tetrachloro-1, 4-benzenedicarboxamide to obtain 2,3,5,6-tetrachloro-1, 4-benzenedicarboxylic acid, effected in the present invention, is carried out in a system consisting of 2,3,5,6-tetrachloro-1,4-benzenedicarboxamide as a feedstock, concentrated sulfuric acid or fuming sulfuric acid and water. Factors influencing the reaction efficiency include the concentration of reactants, temperature, pressure, reaction time, etc. These can be appropriately set individually or in combination.
The concentration of concentrated sulfuric acid in use exerts direct influence upon the reaction efficiency by virtue of temperature and time and the yield of desired product. In the hydrolysis of 2,3,5,6-tetrachloro-1,4-benzenedicarboxamide according to the present invention, concentrated sulfuric acid is considered as not only functioning as a catalyst but also supplying water required for the reaction. The first stage of hydrolysis of 2,3,5,6-tetrachloro-1, 4-benzenedicarboxamide is initiated by an addition of proton to the carboxamido group, and a water substitution reaction follows.
Therefore, in the above hydrolysis, a suitable concentration of concentrated sulfuric acid in use is selected taking into account the proton donating capability of concentrated sulfuric acid and the amount of water that is present prior to and during the reaction. In the present invention, use is made of concentrated sulfuric acid containing water in an amount smaller than a stoichiometric value of hydrolysis. For example, concentrated sulfuric acid wherein scarcely any water is contained, or a mixed acid consisting of sulfuric acid and fuming sulfuric acid, is suitable for use as the solvent for conducting the hydrolysis.
The proton donating capability of concentrated sulfuric acid can be expressed by an acidity function, and the acidity function can be measured by the use of Hammett""s indicator. For example, the acidity function (xe2x80x94Ho) of 95% sulfuric acid is 9.73. When the hydrolysis of 2,3,5,6-tetrachloro-1, 4-benzenedicarboxamide is performed in this sulfuric acid, it is necessary to heat the same at 180xc2x0 C. or higher temperatures for a prolonged period of time. The acidity function (xe2x80x94Ho) of 100% concentrated sulfuric acid is 11.94, and the acidity function (xe2x80x94Ho) of a mixed acid consisting of sulfuric acid and fuming sulfuric acid is greater than 11.94. In the present invention, it is preferred that the acidity function (xe2x80x94Ho) be in the range of 10.27 to 14.44. When the concentrated sulfuric acid of this acidity function is employed, the hydrolysis efficiently proceeds at 110 to 190xc2x0 C. It is still preferred that the acidity function (xe2x80x94Ho) be in the range of 10.57 to 13.00.
Although the amount of concentrated sulfuric acid or fuming sulfuric acid is not particularly limited, it is preferred from the viewpoint of the requirement for desirably maintaining the agitation of solution all of the time during the reaction and from the viewpoint of productivity per batch that 2 to 10 parts by weight, especially 2.5 to 6 parts by weight, of concentrated sulfuric acid or fuming sulfuric acid be used per part by weight of 2,3,5,6-tetrachloro-1, 4-benzenedicarboxamide.
Theoretically, 2 mol of water is needed in the hydrolysis of 2,3,5,6-tetrachloro-1, 4-benzenedicarboxamide. However, as described later, the presence of 2 mol or more of water prior to the reaction causes the hydrolysis to become extremely difficult. For advancing the reaction under the conditions, it is necessary to heat 2,3,5, 6-tetrachloro-1,4-benzenedicarboxamide at high temperatures, for example, 220xc2x0 C. or higher. By contrast, in the present invention, the hydrolysis of 2,3,5,6-tetrachloro-1, 4-benzenedicarboxamide is performed in the presence of water whose amount is smaller than the stoichiometric value of hydrolysis of 2,3,5,6-tetrachloro-1, 4-benzenedicarboxamide. This enables the performance of the above reaction at 110 to 190xc2x0 C. For example, the amount of water which is present prior to the reaction is not limited as long as it is smaller than the stoichiometric value of hydrolysis, i.e., 2 mol. Further reduction of the amount of water is preferred. It is especially preferred that the amount of water be as minute as near to 0 mol. With respect to the water remaining in sulfuric acid, when, for example, fuming sulfuric acid is added, the reaction proceeds leftwards in the following formula so as to form H2SO4 with the result that the amount of water becomes as minute as near to 0 mol: 
Apart from the above, even purified sulfuric acid, in the equilibrium, contains a multiplicity of chemical species, and it is considered that water molecules, although the amount thereof is close to a trace, do exist in the sulfuric acid. Thus, it is presumed that the water produced by the decomposition of sulfuric acid resulting from the rightward advance in the above equilibrium formula (IV) will be supplied to the reaction system in order to compensate for the water consumed by the hydrolysis.
The advance of hydrolysis of 2,3,5, 6-tetrachloro-1,4-benzenedicarboxamide in concentrated sulfuric acid or fuming sulfuric acid according to the present invention is not known at all.
The reaction condition preferably employed for accelerating the hydrolysis at 190xc2x0 C. or below at the initiation or during the reaction is to cause water to be present in an extremely minute amount. Accordingly, it is desirable to use 2 to 10 parts by weight of concentrated sulfuric acid or fuming sulfuric acid per part by weight of 2,3,5,6-tetrachloro-1, 4-benzenedicarboxamide, as aforementioned.
The temperature required for the hydrolysis of 2,3,5,6-tetrachloro-1,4-benzenedicarboxamide is conspicuously varied depending on the amount of water contained in sulfuric acid as aforementioned. Thus, in the hydrolysis of this compound in the sulfuric acid specified in the present invention, it is preferred that the temperature be in the range of 110 to 190xc2x0 C., especially 120 to 185xc2x0 C., and still especially 130 to 180xc2x0 C. When the temperature falls within these ranges, not only does the hydrolysis of 2,3,5, 6-tetrachloro-1,4-benzenedicarboxamide rapidly proceed but also the yield of 2,3,5,6-tetrachloro-1,4-benzenedicarboxylic acid is high.
Contrarily when a large amount of water is present prior to or during the reaction, the hydrolysis would not proceed with practical speed at 190xc2x0 C. or below. For ensuring the advance of reaction in this condition, it is requisite to heat the reactant at severe temperatures such as 220xc2x0 C. or higher. At these high temperatures, it cannot necessarily be assured that the desired compound is produced without the occurrence of undesirable side reactions such as decarboxylation or sulfonation of formed 2,3,5,6-tetrachloro-1, 4-benzenedicarboxylic acid.
Specifically, when the 2,3,5,6-tetrachloro-1,4-benzenedicarboxamide is hydrolyzed at 220xc2x0 C. in sulfuric acid, the formation of by-products attributed to the decarboxylation reaction results in unsatisfactory purity for a technical product for agricultural chemicals or their intermediates. A further serious problem is that heating at 220xc2x0 C. or higher temperatures while using sulfuric acid can be accomplished only by the employment of very special and very expensive facilities to thereby make it difficult to produce the desired compound at low cost.
Therefore, in the hydrolysis of 2,3,5,6-tetrachloro-1,4-benzenedicarboxamide, performing the reaction in concentrated sulfuric acid or fuming sulfuric acid containing water in an amount smaller than a stoichiometric value of hydrolysis is preferred from the viewpoint that the hydrolysis can be effected at relatively low temperature on a practical production level.
The reaction time is influenced by the temperature and the amount of water contained, as aforementioned. Regulating the amount of water contained to a minute level in the initial stage of the reaction and further controlling the reaction temperature at 110 to 190xc2x0 C. is favorable from the viewpoint of industrial production because the reaction time falls within the range of 5 min to 24 hr. Lowering the reaction temperature to below 110xc2x0 C. results in a prolonged reaction time, thereby causing a practicability deterioration.
In the hydrolysis of 2,3,5,6-tetrachloro-1, 4-benzenedicarboxamide in sulfuric acid, 2,3,5, 6-tetrachloro-1,4-benzenedicarboxylic acid is formed and, after the reaction, precipitated out of the reaction system. Further, when the hydrolysis is advanced, for compensating for consumed water, the above equilibrium of formula (IV) is inclined rightwards to thereby produce water. The water produced by the decomposition of sulfuric acid is consumed as a further reactant. Simultaneously; SO3 which would not have direct participation in the reaction is formed in an amount corresponding to the amount of consumed water. In the present invention, the reaction is performed in an open system and is advanced under atmospheric pressure. SO3 formed in the course of the reaction as seen from the formula (IV) is liberated in accordance with the progress of the hydrolysis. Because of the open system, the S03 is released from the system. This further facilitates the progress of the hydrolysis.
In the present invention, in the production of 2,3,5,6-tetrachloro-1,4-benzenedicarboxylic acid from 2,3,5,6-tetrachloro-1,4-benzenedicarbonitrile as well, it is preferred that 2,3,5,6-tetrachloro-1, 4-benzenedicarbonitrile be heated at 110 to 190xc2x0 C. in the presence of sulfuric acid or fuming sulfuric acid containing water in an amount smaller than a stoichiometric value required for forming 2,3,5, 6-tetrachloro-1,4-benzenedicarboxylic acid from 2,3,5, 6-tetrachloro-1,4-benzenedicarbonitrile.
Specifically, in the reaction from 2,3,5,6-tetrachloro-1,4-benzenedicarbonitrile to 2,3,5,6-tetrachloro-1,4-benzenedicarboxamide, 2 mol of water is consumed per mol of the above carbonitrile compound. In the hydrolysis of 2,3,5,6-tetrachloro-1,4-benzenedicarboxamide to thereby obtain 2,3,5,6-tetrachloro-1,4-benzenedicarboxylic acid, 2 mol of water is consumed per mol of the above carboxamide compound. Therefore, theoretically, the total amount of water required for forming 2,3,5,6-tetrachloro-1, 4-benzenedicarboxylic acid from 2,3,5,6-tetrachloro-1, 4-benzenedicarbonitrile is 4 mol.
However, in the present invention, the amount of water required for carrying out the reactions from 2,3,5,6-tetrachloro-1,4-benzenedicarbonitrile to 2,3,5,6-tetrachloro-1,4-benzenedicarboxylic acid is satisfactory, with a proviso that it is smaller than the stoichiometric value required for forming 2,3,5,6-tetrachloro-1,4-benzenedicarboxylic acid from 2,3,5,6-tetrachloro-1,4-benzenedicarbonitrile, i.e., 4 mol.
In the present invention, 2,3,5,6-tetrachloro-1, 4-benzenedicarboxylic acid that is useful as a herbicide precursor can be produced with a high yield by a simple operation comprising hydrolyzing readily procurable cheap 2,3,5,6-tetrachloro-1,4-benzenedicarbonitrile as a feedstock in sulfuric acid (preferably, sulfuric acid containing fuming sulfuric acid) in the presence of water whose amount is smaller than the stoichiometric value of the hydrolysis. Furthermore, in the process of the present invention, from the 2,3,5,6-tetrachloro-1,4-benzenedicarboxamide which, in the presence of water whose amount is not smaller than the stoichiometric value of the hydrolysis, would not be hydrolyzed unless heated at 220xc2x0 C. or higher temperatures in sulfuric acid, 2,3,5,6-tetrachloro-1, 4-benzenedicarboxylic acid can be efficiently produced without the occurrence of side reactions under mild reaction conditions including lower temperatures of 110 to 190xc2x0 C.